Performance improvement is zero.
Reliability gets cut in half.
0/2 = 0.
At one time, I had a Intel X25-M SSD. It worked well, but I needed more space, so I bought a second.
I really wanted a 160gb single image for the "C" drive, so I combined the two in raid-0.
It worked well, but I could not detect any improvement in my user experience.
Later, I used the 80gb drives in other PC's and replaced them with a single X25-M 160gb drive, and performance was equal, if not better.
Lastly, a single ssd at 2x the capacity will not cost as much as a single larger ssd.
You will actually get slightly better performance since there are more nand blocks that can be accessed in parallel.
More importantly, the availability of more free nand blocks improves update performance and ssd longevity.